1. Field of the Invention
The present invention relates to wireless integrated circuit (IC) devices and, more particularly, to a wireless IC device used in a Radio Frequency Identification (RFID) system.
2. Description of the Related Art
RFID systems have been developed as management systems for articles. In the RFID systems, readers-writers that generate induction fields communicate with IC tags (hereinafter referred to as wireless IC devices) that contain information regarding the articles by non-contact methods so as to transmit information. The wireless IC devices used in the RFID systems typically includes wire-wound coils that are formed on substrates on which wireless IC chips are mounted by etching or printing, for example.
Japanese Unexamined Patent Application Publication No. 2000-276569 discloses a wireless IC device formed by rewiring an antenna coil on a wireless IC chip so as to reduce the size thereof. However, since the wireless IC chip itself is very small, the antenna formed on the chip is also very small and has a reduced gain. Accordingly, there is a problem in that the IC chip is not driven unless the IC chip is in very close proximity to a reader-writer.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2000-311226 discloses a wireless IC device in which a wireless IC chip with an antenna coil is mounted on a substrate having a radiation antenna, that is, a wireless IC device having an arrangement in which the antenna of the substrate is opposed to the antenna coil of the wireless IC chip. However, since the antenna of the substrate is a closed loop, it is difficult to achieve impedance matching between the antenna coil of the wireless IC chip and the antenna of the substrate in frequency bands greater than the Ultra High Frequency (UHF) band, although it is possible to achieve a sufficient gain in lower frequency bands, such as about 13.65 MHz.